This commit includes additional non-documentation changes, related to
issues found while documenting it:
- `Simulator.run_until()` no longer accepts a `run_passive=` argument.
Passive no longer exist and in any case defaulting to `False` does not
make a lot of sense from an API perspective.
- `add_clock()`'s `phase=` argument, when specified, no longer has
`period/2` added to it. This wasn't the documented behavior in first
place and it makes no sense to do that.
- `add_clock()` raises a `NameError` if a clock domain does not exist,
instead of `ValueError`.
- `add_clock()` raises a `DriverConflict` if a clock domain is already
being driven by a clock, instead of `ValueError`.
- GTKWave is no longer a part of the installation instructions, and both
Surfer and GTKWave are recommended (in this order).
